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#1
06-26-2012
 USMLE Forums Guru Steps History: 1+CK+CS Posts: 394 Threads: 40 Thanked 218 Times in 99 Posts Reputation: 228
?? Clearance

A subject in whom the inulin clearance is 120ml/min, is administered a,drug whose clearance is 18ml/min. If drug is 40% plasma protein bound then what percentage of Filtered drug must be reabsorbed in the renal tubules?

A) none
B) 12.5
C) 25
D) 50
E) 75

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#2
06-26-2012
 USMLE Forums Master Steps History: 1+CK+CS Posts: 1,039 Threads: 189 Thanked 557 Times in 318 Posts Reputation: 567
.

ok, so inulin clearance is basically GFR.

GFR = 120 ml/min so 120 ml/min of plasma is being cleared from some substance by the kidneys. Protein-bound substances are NOT filtered so only 60% of that 120 ml of plasma contating that drug will be filtered, so 120 ml/min x .06 = 72 ml/min - and that's in theory.
BUT...
In practice we know from the stem that only 18ml/min is being cleared from that drug. So it means that some fraction of drug has been reabsorbed in the tubules.
so now we see that 72 ml/min has been cleared at the level of glomerulus but only 18 ml/min was actually cleared at the end of the tubule journey 72 - 18 = 54 - that's the amount that has been reabsorbed. so now we calculate what fraction of initially cleared plasma is the reabsorbed amount.
54/72 = 75%.

so E) is the answer here.
 The above post was thanked by: modesty (06-26-2012), Rookie (06-26-2012), Zamalek (06-26-2012)

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